Crystal Ladder Filter 20Mhz

Minima_Filter

Well I had a lot of fun building the Minima 20Mhz crystal filter. There is no shortage of good information on the topic of crystal ladder filter construction scattered around the Internet. So I’ll try not to tediously duplicate but rather focus on what I learnt specific to the Minima. For those starting from point zero here is just one link to a document containing a reasonable summary of the current state-of-the-art:-

Crystal characterization and crystal filter design.

The first thing of import I learnt about the Minima filter is that it is a QER filter. QER being short for “Quasi-Equiripple”. The QER filter topology is easily distinguished by the use of the two parallel crystals at either end of the filter. This filter type is attributed to none other than David Gordon-Smith G3UUR. Who also developed what is most likely the most popular method for measurement of crystal motional parameters, the G3UUR shifted frequency method.

What I found out about QER filters is that they are “relatively new”. So there is not a whole lot of information about them around the Net. They are a variant of the Min-Loss Cohn Filter topology which substantially reduces unwanted passband ripple. Which is a very good thing. Min-Loss Cohn is popular for simple QRP designs because it is simple. As all of the capacitors in the network have the same value. The only thing wrong with the Cohn filter is the rather large ripple in the passband. QER maintains the simplicity but substantially removes the ripple.

Vk2sja-Photo39

So I decided that I would use the G3UUR shifted frequency method to characterize my crystals. Which seemed appropriate given that the filter was a QER filter. So step one was to build a G3UUR test oscillator and measure two frequencies per crystal for each crystal in my set of 42. Above you can see the test oscillator and frequency meter in action. It is going to be difficult for some to find a counter that will give 1Hz accuracy at 20Mhz. But I’d suggest that the exercise is still worth-while even if you have to scale back to a 10Hz resolution.

Vk2sja-Photo40

So I sorted and labeled each of my crystals with a number from 1 to 42 and then proceeded to measure them. During the process I found three crystals from this batch that wouldn’t even start in the test oscillator. Not quite sure what would have happened if I had managed to select one of these for inclusion in a filter at random. But I’m thinking only bad things would likely come of it.

What you’re trying to do here is pick a matched set of crystals that oscillate at very nearly the same frequency, or as close as you can get them. The data was all entered into LibreOffice Calc spreadsheet and with a bit of software magic the crystals were all helpfully sorted. Such that picking a group of 8 crystals with the closest possible frequency span could be done at a glance.

If you’re not planning on doing the whole “crystal characterization” filter design thing. Then I’d still recommend that you at least do this step. You could use the Minima BFO as the test oscillator. If nothing else it will remove faulty crystals from the equation and it is much more likely that you will end up with a crystal filter instead of a crystal brick wall.

Vk2sja-Photo41

So I went ahead and built the crystal ladder filter that you can see above with my “matched” crystals. Oddly enough I (foolishly?) chose to build this filter arbitrarily with the 100pF capacitors as specified in the Minima circuit diagram. Partly because I wanted to see what the result would be. But mostly because at that time I was still trying to get my head around the software design process.

Obviously if you have gone to all the trouble of properly characterizing your crystals then the objective would be to feed that data into some design Software. Requesting it to give you a particular desired bandwidth for your new filter based upon the motional parameters you have carefully collected. The software should then tell you (predict) the inter-stage capacitor values (all the same value for QER) and the filters characteristic impedance. All hopefully with a high degree of accuracy. So just grabbing five 100pF capacitors and slapping them in like I did is somewhat missing the point entirely.

Vk2sja-SA-Shot6

So the resulting filter was connected up to the Rigol DSA-815 and a plot emerged. I have to say I was pretty pleased to see this picture. But what’s this, the 3dB BW is only 1.9kHz? Others have been reporting 4.5, 6 and even higher 3dB bandwidths! Which just goes to show that the resulting bandwidth is very dependent upon the actual crystals being used and their motional parameters.

Filter1_20Mhz_1k9BW

The plot above is the same 1.9kHz filter. It was generated with the vna/J software using a MiniVNA-Pro. This plot shows both the S21 transmission loss plot and the S11 refection or return loss plot overlayed. You can see more detail from this instrument as the minimum bandwidth resolution of the Rigol DSA-815 is only 100Hz. But the Rigol is more than up to the task of providing crystal ladder filter sweeps for Amateur purposes. The return loss plot looks pretty messy and not anything like the ones you see in text books. To be honest I don’t really have a good feel for how good or bad this filters return loss is. Time, more reading and some experimentation will reveal all.

Dishal

Along the way I had discovered, to the best of my Internet sleuthing, that there is currently only one software design tool which will allow you to predict and model the QER filter. And that software package is called Dishal.

So I fired up Dishal as seen above and proceeded to try to understand the inner workings and why my filter was so narrow compared to those built by others. I’m going to use Dishal to predict the required capacitor values for a filter 3dB bandwidth of 2.7kHz. Given that I tell Dishal what -my- specific crystal motional parameters are.

I should mention that QER filters with all capacitors being of equal value makes experimenting easy. It wouldn’t be hard to arrive at a desired filter bandwidth simply by logical substitution of the capacitors. Reducing the capacitor value(s) increases bandwidth while increasing capacitor value(s) decreases bandwidth. So with some educated guess-work and only a few wholesale capacitor bank change outs. You should arrive pretty close to where you want to be in terms of bandwidth. Of course you’re not going to know what your filter impedance is until you test/measure it. Which is the other very important thing that your filter design software will tell you. But it can all be done. So even if you don’t have any method of sweeping a filter then you can still build them!

The first thing one needs to get your head around with QER and Dishal is that nearly all of the program functionality displayed above, including the ability the plot pretty filter plots. Is not applicable to QER filters. If you’re using this software to design a QER filter, then you are simply going to use two or three program features available from the drop-down menus at the top of the screen. The option “Xtal”, and then later in the design process the “QER (G3UUR)” option. Likely followed by the “LC-Match” to assist in building a matching network.

Under the “Xtal” option you get two sub-options for the two most popular methods for performing Crystal Characterisation of the motional parameters. The G3UUR method and the 3dB BW method. I used the G3UUR method but later cross checked with the 3dB BW method. I found that either produced results so close that it wouldn’t matter which you use. So the selection of method will be something of personal preference. Perhaps dictated by the test equipment you own. If you’re lucky enough to already own a VNA then the 3dB BW method may well be easier.

QER_Calculator

After some trial and error I discovered that Dishal could be used to accurately predict the capacitors required to build a filter of a given bandwidth. What I found however was that the crystal holder capacitance value was critical. I had to measure the capacitance value between the two crystal leads, record this value. Then I needed to short the two leads together and measure the capacitance between both the shorted leads and the metal case of the actual crystal. The second amount then needed to be deducted from the first and the result is entered into Dishal as the Cp (pF) figure. It was only the difference of a couple of “Puffs” but it made a world of difference to the prediction.

So after carefully collecting the crystal frequencies(s) of all my crystals. Then selecting the 8 crystals closest together in band-spread. Only 65 Hz spread between them. Then measuring the holder capacitance of all these eight crystals. And then averaging all of these eight values to arrive at a single value to enter into Dishal for each parameter. Dishal then proceeded to predict 65pF as being required for a 3dB bandwidth of 2.7kHz with some 94 ohms of in/out impedance. I substituted a standard capacitor value of 68pF and went ahead and built my second filter. I had decided to keep the 1.9kHz 3dB BW filter. After all it was a good looking filter!

And the result?

Filter2_20Mhz_2k7BW

Again the plot above was produced by vna/J and a MiniVNA-Pro. Using the marker math function of vna/J it was revealed that the 3dB bandwith was:-

19,997.524 Low
20,000.175 High

Actual 3dB bandwidth = 2.651 kHz! Not too bad a result for 2.7kHz requested.

A Dishal help file warns that calculations are based on mathematically perfect components. Which of course don’t exist in the real world. The resulting losses mainly in the capacitors leads to a slightly smaller 3dB bandwidth than that calculated. The suggestion is to work out what your desired 6dB BW would be and then enter that as the requested 3dB BW (i.e. lie to the program essentially). This will apparently place you *very* close to your actual desired BW. The fact that I did not do this would go a long way towards explaining my smaller than requested BW.

So I was very happy with the new filters width. But what’s that wavy pattern across the top of the passband? Ripple! Nearly 3dB of it at a quick glance at the plot. Now I asked the question:- “How much ripple is acceptable?” and basically the answer came back:- “How much are you prepared to put up with?”. I even found a commercial manufacturers technical bulletin talking about adjusting a filter matching network which said that anything “less than 3dB” of ripple was to be deemed acceptable. Admittedly this alignment procedure came from the valve era and I think our standards today are a little higher, even for the home-brewer. Suffice to say that 3dB is probably too much and if you have it down to less than 0.5dB ripple then that is considered very good indeed.

So where did the ripple come from? The ripple from a practical stand point is caused by two things. Failure to match your crystal filters input and output impedance properly and how wide your filter is. The wider the filter, the more ripple it is going to have. This means that building an exotic narrow CW filter is actually pretty easy. A 400Hz CW filter using the traditional Cohn topology is probably not going to have any significant passband ripple. But if you try to build a filter with wider SSB bandwidths, like the 2.7kHz 3dB BW filter above, then keeping the ripple under control gets harder, much harder.

It also goes to show how lucky I was with the first filter I’d built. The bandwidth may have been an unexpectedly narrow 1.9kHz but the combination of narrow BW and what must have been a filter with very, VERY close to 50 ohms input and output impedance resulted in a very flat passband.

What to do to reduce the ripple? I needed to impedance match the input and output of my new 2.7kHz filter to 50 ohms.

Vk2sja-Photo42

Now Dishal had predicted about 94 ohms impedance in/out. So I guess I could have just matched to that. But I had altered the capacitor values slightly. By now I had learnt that sometimes very small values of capacitance make big differences. I guess I could have used Dishal, and altered the requested bandwidth to arrive at the actual value of capacitance I had used and then read off the predicted Zo in/out. But I wasn’t sure if this approach was valid. So in the end I went and built a couple of in-line resistive terminators.

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You place these either side of the filter. Then adjust the mutli-turn pots for best passband response while looking at the spectrum analyzer filter sweep in real time. The resistive terminations cause bucket loads of additional insertion loss but you don’t worry about that. This adjustment is subjective and a bit tricky. As I had to keep adjusting the DSA-815 settings to keep the top of the passband visible. Adjusting the resistors tended to make important parts of the trace disappear off the bottom of the analyzer screen. Eventually I arrived at what I thought looked like the flattest passband response. And when the terminators were removed and the actual resistance checked with an Ohm meter I was pretty close to 75 ohms.

So I then asked the Dishal LC-Match to give me the values for a L-Pad to match 75 ohms to 50 ohms. Which gave series inductance of 0.28uH with parallel capacitance of 75pF. I substituted 75pF for a standard value of 82pF and with a bit of help from the AADE LC meter (fantastic tool this!) built some 0.28uH air-core coils and the result looked something like this:-

Vk2sja-Photo43

And the actual filter response now looked like this:-

Filter2_20Mhz_2k7BW_matched

Filter2_20Mhz_2k7BW_matched_closeup

~ 0.5dB passband ripple!

As Colonel Hannibal from the “A-Team” would say:- “You gotta love it when a plan comes together!”

73, Steve.

More on FET Matching

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Well as promised here is a quick run down on my journey through the FET matching process. Pictured here is my latest mixer and above it are three different little test jigs that I built to help match and measure FET characteristics. From left to right we have a FET Voltage at Pinchoff (VP) test tool, a FET matching bridge and at far right a very simple jig for measuring FET IDSS. It simply shorts the Gate and Source together.

All of the information for building these devices started at the web site of QRP Homebuilder. Sadly in recent days this outstanding treasure trove of electronic goodness has been pulled from the Internet. Thankfully Todd Gale, VE7BPO the owner and creator of QRP Homebuilder has archived the entire site and made it available as a single (large) PDF document. See here for details.

I found measuring IDSS is a pain because as the current flows heat is caused and circuit conditions change. This in turn causes the meter readings to change before your very eyes as you watch it. I’ve read that manufacturers actually pulse DC current into the FET to stop this from happening when they test for IDSS. I have mentioned in a previous post that I settled for simply switching the current flow on. Waiting a set number of seconds (10 in my case, the same number for each test) for things to settle down a bit and then simply recorded the number displayed on the volt meter at that point in time.

The FET matching bridge is very precise. It tells you when two FET’s are exactly the same. Which is pretty much what you want for a FET mixer of any type, be it Passive, Active or whatever. But this does not tell you what the pinch-off voltage is (VP). Which could be very important if your trying to exceede VP deliberately for the purpose of creating a switching or “chopper” mixer. The Minima mixer is supposed to be a mixer of this type. Equally, if you wanted to build a mixer that remains in the non-linear square-law region. Then the reverse would become important. You would want to make sure your mixer was NOT exceeding VP.

So that’s what the most complicated test jig pictured above is for. The FET Voltage at Pinchoff (VP) test tool. This tool allows you to adjust the FET bias such that you just hit pinch-off (VP) when your digital volt meter reads zero volts across a resistor. Once this is done you then connect your volt meter across another set of test points in the circuit to read off the actual pinch off voltage (VP).

Again all of the knowledge I gathered together to build and use these tools came straight from the QRP Homebuilder web site. Which is now archived into a PDF linked above.

One final comment.

What I found (like Professor Vasily Ivanenko before me) was that if two FET’s have matching IDSS values then the VP of those same two FET’s would be very close indeed. So I think for all practical purposes that Amateurs can match FET’s for use in mixers by simply using the most simplest of the test jigs above. The IDSS test tool. So simple in fact, it does not really need a jig at all. It is only a time saving convienience if you plan on testing multiple FET’s in the one session.

So this then will get you a “matched” set of FET’s. Having got that far then all you the need to make sure that your mixer circuit is well and truely exceeding VP, or not. Depending on the mixer type your trying to build. For the Minima it is supposed to be a switching or “chopper” mixer. As such VP needs to be exceeded. I checked mine and it was. But there is some general concern about that the original Minima mixer design may not be able to deliver sufficient voltage swing to drive the J310 FET’s into cut-off. This does not mean that the mixer would not function. In fact it may even mix quite well. But it would not be operating as a high performance commutating (switching) mixer. Which has implications for the rest of the circuit as a whole.

Next up, some crystal filters…

73, Steve

Mixer Melodies. KISS, KISS V2 and Double-KISS

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Well its been a while between posts. I got tied up experimenting with Mixers for longer than expected. Here you can see a collection of the various mixers I’ve built over the last few weeks. A Passive Quad J-FET Mixer, Two versions of the J-FET KISS mixer and a couple HC4066 CMOS switching mixers. Many of these went through two or three re-builds while playing around with various configurations.

So why all the different mixers?

Well the story goes something like this…

KISS_V2

Early in the life of Minima some builders started reporting high levels of local oscillator leakage to the RF port in TX mode. You can see this at the 34Mhz point in the spectrum analyser screen shots above. The solution? Farhan then offered the KISS V2 circuit for testing. This circuit gave extremely good adjustable LO suppression but some experimenters, including myself, started seeing higher levels of insertion loss with this mixer. The original KISS mixer had about 7dB insertion loss while the newer KISS V2 was being reported at 10dB+. The first version of KISS V2 I built was up around 14dB. By the third re-build I did get it down to 10dB but couldn’t manage to get it any lower than this.

To me the large local oscillator (LO) leak seems inherent in the design of KISS V1 and other variant mixers based on this pattern. If you look at the circuit schematic you can see that the LO (if nicely formed) will be of equal but opposite magnitude in each side of the input into the mixing transformer. As such these signals should cancel out very nicely at the mixing transformer centre tap. However not so much attenuation is available to the other winding of this transformer. Both the DSA-815 screen shots above are looking at the output from the port on the second mixing transformer winding, not the centre tap port. Hence the high LO level at 34Mhz.

Vk2sja-KISSv3

In all this experimenting I did a lot of Googling slash reading and came across this paper:-

RF Mixer Design

On page 16 I found “Figure 28: Circuit diagram of a FET based switching mixer”

So on a whim I decided to blend this design with the alternate bias method that Joe outlined. This modified circuit can be seen left. I’m now somewhat presumptuously calling this J-KISS version 3. Because that’s easier than typing “Modified version 1 Minima J-KISS mixer with Joe’s floating bias modification and a couple of extra resistors that I added just to see what would happen…”, all the time.

Vk2sja-Photo36

Well it turned out that it worked rather well. In fact for me it worked better and more consistently than any other version that I tried building. Joe’s floating bias had pretty good conversion loss already but adding the resistors dropped the unwanted local oscillator noise almost into the noise floor.

Vk2sja-SA-Shot3

So here is a couple of screen shots of KISS V3 in action. To the left is the mixer in the transmit direction. While on the right is the same mixer running in the receive direction. Note that this clearly shows the difference in the LO leak level depending in direction of signal flow (which port you’re looking at). Which is high here in the receive direction. This characteristic was true of all the J-KISS mixer variants I built.

Vk2sja-SA-Shot4

A couple of important things we should note about this high LO leak level. First, in the original Minima circuit the mixer was connected the other way around. So this higher LO level would be present during transmit and flow through to the the low pass filters. Simply reversing the direction of signal flow through the mixer as done here with J-KISS V3 (and as Farhan intended for J-KISS V2) will now present this higher level to the crystal filter instead. The idea being the the crystal filter will do a better job of filtering it out than the low pass filter. Will this cause any receiver issues? I don’t know yet but at least the transmitted signal should now hopefully be clean and within legal limits for harmonics.

The second important thing of note is that in those J-KISS mixer variants with some sort of manual bias adjustment. Which include both of Farhan’s J-KISS V1 and V2 designs (although the bias arrangement is very different in each). The adjustment will make a significant change to the LO leak level but only on the port connected to the centre tap of the mixing transformer. On the port connected to the second winding I hardly noticed any change at all. Just a few dB at most.

Now my J-KISS V3 mixer was looking pretty good. And while performance is nothing to get too excited about. Let’s face it, a cheap Mini-circuits SBL-1 double balanced diode mixer module would probably run rings around it. But at least my mixer was behaving like a proper mixer should and I can say I built it myself! Oh, and it’s also operating as a switching mixer or “chopper” which is important in the Minima. This version was in some ways the easiest to build and get going because it has no bias adjustment. But herein lies the catch…

Vk2sja-SA-Shot5

For this mixer to work well the J310 FET’s must be very well matched prior to building the unit. So what happens if you don’t? The spectrum analyser screen shot to the left shows exactly the same mixer used above but with a pair of deliberately mismatched J310’s installed. We can see that all the unwanted signal levels jump upwards. The dreaded LO leak level alarmingly so. Interestingly the conversion loss seemed to remain largely unaffected. Compare this screen shot with the one two above.

So building this mixer means that you must match your FET’s. Personally having now been through the FET matching exercise I don’t find the task too onerous. No more difficult than matching the diodes we typically use in other mixers. A procedure which we Radio Amateurs take for granted. And probably a lot easier than profiling/matching crystals for ladder filters.

It occurs to me that the FET matching process only really requires an accurate digital multimeter. While the adjustment of a mixer balancing pot ideally needs a Spectrum Analyser. Admittedly you could probably make do with a general coverage receiver listening to the LO frequency via a direct cable connect to the mixer with some in-line attenuation. The multimeter seems the somewhat simpler though. Anyway it could be that for the average home builder pre-matching the FET’s used to build a mixer may be in fact be easier than adjusting a balance control setting after the mixer is built. Perhaps, maybe…

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So finally I tired of playing with mixers. There is much, much more that I have not tried and should have. Looking back there are experiments which need to be re-done because of flaws in my methodology or understanding at that time. Please understand that what I present here are just my experiences. While I stumble around learning new skills. This is what happened to me. With the mixers that “I” built! I’m no Electrical Engineer or RF Design Engineer. Just a hobbyist Amateur Radio enthusiast. So your mileage, as they say, may vary – significantly! But if this project was ever going to reach an operational state. Then progress had to be made at some point. At a later date since this build is a modular scratch build with no PCB to lock me into a particular design. It will be easy enough to swap out the mixer module with an improved version.

So enough was enough and I installed my modified J-KISS in the radio. As seen here from the underside. No connection to either the IF or RF ports as yet. The coaxial cable from the LO port can just be seen sneaking its way out and immediately up to the Si570/logic PCB topside.

Double_KISS

On a final note this section would not be complete without mention of the “Double KISS”. Farhan released this circuit to the Minima group for experimentation. Called the “Double-KISS”, no doubt because it is doubly balanced instead of singly balanced like the original Minima JFET KISS… I think? This mixer uses a modern high speed CMOS bus switch at its heart. Which brings it much closer to the original KISS design detailed in the “Mixer Musings and the KISS Mixer” paper by Chris Trask/N7ZWY. Which is where all this mixing business started in the first place.

So a lot of experimental energy is now being invested in this direction. This style mixer promises exceptional performance. The only downside being the somewhat exotic part required. They are not particularly expensive, just not available from your local electronics store. So the Minima continues to evolve as time goes by. For myself I have ordered some suitable high-speed CMOS bus switches for experimentation but at this stage plan on completing the radio close to the original design. I can then evaluate its performance before making any changes like a mixer substitution.

Well that’s all for the moment. Next up I’ll back-track a little and talk about JFET matching in more detail. Then we shall tackle the 20Mhz Crystal Ladder Filter.

73, Steve. VK2SJA